a. Field of the Invention
The present invention relates to crystal growth, and more particularly it pertains to solution growth of compound semiconductor mixed crystals containing at least one constituent volatile element, through external application of a controlled vapor pressure of said constituent volatile element.
b. Description of the Prior Art
Various methods have been proposed for growing semiconductor crystals. Such various methods can be roughly classified into the melt growth method and the solution growth method. Melt growth is advantageous in respect of high growth rate, but the inherent properties of the substance to be crystallized, such as melting point, viscosity, etc., impose various limitations to the crystal growth. Solution growth, on the other hand, provides for the advantage that the properties of the substance can be modified in various ways such as lowering the growth temperature, lowering the viscosity, etc.
Higher growth temperature gives rise to more lattice defects, higher dissociation pressure, and the like as well as the disadvantage that said higher temperature, itself, may bring about practical problems including the desinging of heating means. Therefore, from the point of perfect crystallography, it is desirable to grow crystals at a temperature as low as possible, and therefore, the solution growth is very advantageous in this respect. However, deposition of crystal from a solution would lead to compositional changes of the solution. Compositional change of the solution should change various parameters of the crystal growth.
Solution growth of the temperature difference method (TDM) proposed by the present inventor is very advantageous in that the growth temperature can be lowered by the use of a solution, and the composition of the solution can be kept constant by continuously dissolving source material(s) to the saturation concentration at the higher temperature portion. It may be compared with the floating zone growth method wherein a molten zone runs along a source ingot.
For growing a compound semiconductor crystal such as III-V compound or II-VI compound semiconductor crystal, there exists another problem, i.e. a high dissociation pressure which leads to evaporation of a volatile component and hence deviation from stoichiometry. Deviation from stoichiometry first introduces lattice defects since there is a deficiency of one component. If the deviation from stoichiometry is large, there may be no chance of growing the desired single crystal. For suppressing such evaporation of a volatile component and hence deviation from stoichiometry, application of vapor pressure of the volatile component is recommended. Usually, it has been considered that application of the equilibrium pressure is sufficient to keep the stoichiometry.
Usually, however, little care had been taken to precisely control the vapor pressures during crystal growth, excepting those for preventing evaporation or dissociation of source material.
Liquid encapsulation may be used for preventing evaporation of a material located at a high temperature zone as well as for protecting it from contamination. Liquid encapsulation of GaP with B.sub.2 O.sub.3 or BaCl.sub.2 layer was proposed (J. Crystal Growth 3, 286 (1968)). However, evaporation cannot be completely avoided by such liquid encapsulation, and some part of said material located at said high temperature zone evaporates away through the covering layer. Furthermore, the covering substance may be introduced into the grown crystal.
In U.S. Pat. No. 3,902,860 to Akai et al., compound semiconductors having a high dissociation pressure are produced without the use of a sealed reaction tube and under a controlled vapor pressure of a volatile component contained therein. The vapor of a volatile component is arranged to bubble through a molten source material while maintaining a pressure balance between an enclosed chamber and an outer chamber for preventing destruction of the enclosed chamber. The pressure is adjusted to a level lower than the equilibrium pressure.
Experiments by the present inventor (and his colleagues) have revealed that application of such an accepted equilibrium pressure is not sufficient to achieve the correct stoichiometry and perfect crystallization.
On the other hand, there also has been proposed a theory that the stoichiometric composition of a crystal can be spontaneously achieved throughout the crystal growth only by controlling the temperature of the melt to produce a desired pressure of a constituent component, because the pressure of the constituent component would be automatically determined by the temperature of the melt.
Further, according to the solution growth method, the vapor pressure of a constituent volatile element can be reduced due to lowering of the growth temperature and also due to reduction in the molar ratio of the solute source material in the solution.
Therefore, the relation of stoichiometry in a melt or solution to the vapor pressure of volatile elements has not been seriously considered and not analyzed in detail. It has been found, however, that the compound semiconductor crystal produced by the conventional method has a large deviation of composition from stoichiometry.
With the increase in industrial demand and the progress in crystal growth technique, it has become necessary to produce reliable and long-life semiconductor devices by using crystals of higher quality and higher purity. It is considered that the most serious problems in compound semiconductor crystals are caused by deviation from stoichiometric composition, for example, the crystal lattice of III-V compound GaAs must consists of the same number of Ga and As atoms. That is, the last and greatest barrier in the production of crystals of highest quality is to avoid such deviation from stoichiometric composition.
The present inventor has found that deviation from stoichiometry in a compound semiconductor can be controlled precisely and can be minimized by precisely controlling the vapor pressure of a volatile element employed, and that the vapor pressure of at least one volatile element must be precisely controlled for achieving stoichiometry in the crystal produced. The vapor pressure to be applied should be a function of temperature employed. The optimum vapor pressure has been found to be far greater than the accepted equilibrium pressure, especially at lower temperatures.
For growing a perfect compound semiconductor crystal, the present inventor has proposed the temperature difference method under controlled vapor pressure (TDM-CVP), which is basically a method of growing crystals at a fixed temperature under controlled vapor pressures for obtaining stoichiometry in a compound semiconductor (e.g. J. Crystal Growth 31, 215 (1975)). The growth temperature can be reduced by the use of a solution. An extremely high-quality crystal can be grown at a fixed temperature and at a fixed rate by establishing a temperature difference in the solution and by dissolving a source material continuously up to the saturation concentration in a high temperature portion of the solution. Further, a precisely controlled vapor pressure of a volatile component is applied externally on to the surface of the solution to thereby minimize deviation from stoichiometry. The optimum vapor pressure to be applied to the solution is determined from the measurement of an appropriate physical property of a grown crystal as a function of applied vapor pressure.
Mixed crystals, especially compound semiconductor mixed crystals, are attracting the attention of those concerned for the reason that they provide for semiconductors of arbitrary band gap, etc. The growth of mixed crystals, however, is not always analogous to that of elementary or compound semiconductors since mixed crystals require microscopic chemical (or compositional) disorder and yet macroscopic chemical uniformity. Behavior of mixed crystals is not always intermediate of terminal materials. Further, growth of a mixed crystal usually occurs as being of a composition different from that of the source, so that the grown crystal may have a compositional gradient even when stoiciometry is attained. Attainment of stoichiometry, microscopic chemical disorder and microscopic uniformity is considered to be the most important problem for growing mixed crystals.